Healthy people typically spend approximately one third of their time sleeping. People of what may be considered less than optimum health spend greater amounts of time reclining. Beds of various forms have been developed in order to provide comfort to the user. This is particularly true of patients in hospitals and health care facilities, as well as those in homes who, for various reasons, are bed ridden.
Once one is in bed for extended periods of time in a situation or condition which does not allow movement in order to maintain comfort, complications may develop. This is typically in the form of what are generally referred to as bed or pressure sores, or more specifically referred to as decubitus ulcers.
When a person is lying in a fixed position, the weight of the person as carried by the person's skeleton presses against the underlying tissue. If this pressure is great enough, the flow of blood to and from the tissue may be cut off. The arterial capillary pressure is generally understood as being about 30 to 35 mm Hg. The venous capillary pressure is about 10 mm Hg.
It has generally been understood that if the maximum pressure on the skin, sometimes referred to as the interstitial skin pressure, is reduced below the arterial capillary pressure the tissue will receive adequate blood flow and bed sores will be prevented. See for instance, Meer, "The Tissue Therapist's Guide to Understanding Skin Destruction", Hospitals & Healthcare International, September/October 1983, two pages; "The Body Fluids and Kidneys", Textbook of Medical Physiology, Seventh Ed., Edited by Guyton, W. B. Saunders Co., 1986, p. 354; Stewart, "Why 32?", Pressure Ulcer Forum, Vol. 2, No. 2, Spring 1987, Gaymar Industries, Inc., pp. 1-2; and Agris et al., "Pressure Ulcers: Prevention and Treatment", Clinical Symposia, CIBA, Vol. 31, No. 5, 1979, pp. 2-9.
It will be seen in reviewing these articles that it is widely accepted that bed sores originate at the skin surface and then spread inwardly. This is based on a corresponding understanding that the pressure is at its greatest at the skin surface.
As a result of this general understanding many forms of patient support systems have been developed. This development has taken two general directions. The first is in the area of the framework and platform which supports a cushion or mattress. The other is the form of the cushion positioned on the platform on which the patient is directly supported.
A bed frame which can be moved into various orientations and configurations can alleviate some of the pressure problems discussed above. For instance, a bed which tilts from side to side or from head to toe can be used to change the general skin surface area on which the patient is supported. Other beds variously control the position of the lower legs, upper legs and torso of the patient.
Incorporated with these concepts are the needs of patient-caring personnel, such as nurses, who must be able to gain access to the patient as well as manipulate the patient for entry onto and exit from the bed or for treatment while in the bed.
Examples of known beds which provide a framework or support which permits manipulation of the bed platforms include the following U.S. Pat. Nos.: 3,220,020 to Nelson for "Adjustable Height Bed"; 3,434,165 to Keane for "Hospital Bed"; 3,462,772 to Morrison for "Center-Pivoting Bed"; 3,611,452 to Turko et al. for "Invalid Bed Construction"; 3,611,453 to Lokken for "Invalid Bed and Tilt Actuating Mechanism"; 3,644,945 to Goodman et al. for "Adjustable Beds"; 3,678,519 to Szucs for "Hospital Bed"; 3,724,004 to Behrens for "Adjustable Bed"; 3,733,623 to Croxton for "Hospital Beds"; 3,900,906 to Berthelsen for "Adjustable Bed"; 3,997,926 to England for "Bed with Automatic Tilting Occupant Support"; 4,025,972 to Adams et al. for "Elevating and Trendelenburg Mechanism for an Adjustable Bed"; 4,099,276 to Hunt et al. for "Support Appliances Having Articulated Sections"; 4,356,577 to Taylor et al. for "Multipositional Medical Bed"; and 4,371,996 to Nahum for "Articulated Bed".
A review of these references discloses generally complex mechanical structures used to provide the desired functions. This mechanical structure prevents the bed from being sufficiently manipulatable to achieve all orientations desired. A more versatile design is disclosed by Berthelsen in the '906 patent. In this patent, the bed platform is described as being supported centrally on a universal joint having a multilegged support. Four hydraulic arms spaced from the universal joint provide pivoting about two axes intersecting at the universal joint. Suggestion is also made that the central frame supporting the universal joint could be built on an intermediate frame, the elevation of which is adjustable.
This device thus provides a simplified design. However, movement is limited to pivoting about two axes to vary the pitch and roll of the platform, as well as height adjustment. However, a platform cannot be positioned in all desired orientations. For instance, the bed requires a fixed platform frame. Thus, variations of the platform itself, such as is disclosed by Behrans, are not possible. Further, the platform cannot be positioned in a near vertical orientation, as substantially provided by Keane, England and Taylor et al.
Some of these references also disclose apparatus associated with the bed to restrain a patient while accommodating patient care. A specific example is the guard rail design disclosed by Nelson in U.S. Pat. No. 3,220,024 for "Bed Site Guard Rail". Such guard rails typically consist of a metal tube loop positioned vertically on the side of the bed and structured to swing down when a lock is released. Such devices do not facilitate the attachment of patient treatment apparatus. Separate support stands or specially designed beds must then be used. Such rails also do not permit the patient attendant to approach the patient any more closely than the side of the bed, thereby requiring the attendant to bend over the bed to reach the patient, putting a strain on the back of the attendant.
Regarding cushioning systems, the main focus of commercial or other known conventional beds has been to provide either a uniform low pressure surface or an alternating pressure system. A uniform system is provided by what is referred to as an air fluidized bed such as are sold commercially under the names Clinitron by Support Systems International, Inc. of Charleston, S.C. and Skytron of Grand Rapids, Mich. These systems are described by Hargest in "Problems of Patient Support: The Air Fluidized Bed as a Solution", pp. 269-275.
A substantially uniform system is provided by what are referred to as high or low air loss bed systems. See for instance the article by Scales entitled "Air Support Systems for the Prevention of Bed Sores", pp. 259-267. Such beds are sold under the name Mediscus Products Limited of Wareham, England and are described further in U.S. Pat. No. 4,525,885 for "Support Appliance for Mounting on a Standard Hospital Bed". Other similar commercial products are sold under the name Flexicair by Support Systems International, and by Kinetic Concepts, Inc. under the name KinAir and recently, a company called Airplus. These beds typically have a plurality of sets of air sacs, each set corresponding to a longitudinal section of a patient's body. The air is pumped through a pressure-compensating valve into the sacs to achieve a desired pressure. On some units the air bleeds through the fabric of the sacs to keep the patient's skin dry. This system is also described in U.S. Pat. No. 4,525,885 issued to Hunt et al. for "Support Appliance for Mounting on a Standard Hospital Bed", assigned to Mediscus Products Limited.
A test and comparison of the fluidized bed support system and the low air loss bed system is presented in "The Effectiveness of Air Flotation Beds", Care Science and Practice, November 1984, two pages. This study shows that both systems provide a range of pressures between 15 and 32 mm Hg. Thus, an overall, uniform low pressure is maintained.
A third common type of system which has recently been developed is what is referred to as an alternating pressure system. This system generally consists of two layers of air cells which typically extend the length or width of the bed. In such a system referred to as a large cell ripple mattress, the lower layer is maintained at a constant pressure with alternate cells in the upper layer being inflated. Periodically, the other set of alternate cells are inflated and the original set deflated.
A more recent variety provides an intermediate form of cycling in that the cells are pressurized to varying degrees, with the pressure of inflation shifting periodically down the length of the bed one cell at a time. Thus, an air wave of very low frequency is produced. This air wave is produced in both upper and lower layers of cells with vertically aligned cells being inflated a like amount. This system is described in U.S. Pat. No. 4,225,989 issued to Corbett et al. for "Beds and Mattresses", and is compared to the alternating pressure large cell ripple mattress in Exton-Smith et al., "Use of the `Air Wave System` to Prevent Pressure Sores in Hospital", The Lancet, June 5, 1982, pp. 1288-1290. This study was qualitative in nature and found the air wave system more effective, but one which did not eliminate the development of bed sores once they had started.
There are four companies producing vinyl overlay mattresses which variously provide alternating pressure with two alternating circuits of pressure cells. Grant of Stamford, Conn. uses longitudinal cells the length of the mattress. NOVA Health Systems, Inc. of South Easton, Mass. uses lateral cells. Gaymar of Orchard Park, N.Y. and Huntleigh Technology of Manalapan, N.J. use lateral sets of oval cells. Each circuit of pressure cells has the same pressure throughout at any given time.
A simplified air wave system is described in U.S. Pat. No. 4,225,989 issued to Corbett et al. for "Inflatable Supports". The mattress disclosed in this design has a rippling effect produced in the upper layer, with the lower layer a single uniform cell.
Other representative proposed systems are disclosed in the following U.S. Pat. Nos.: 3,893,198 to Blair for "Mattress for Preventing Bedsores"; 4,224,706 to Young et al. for "Pneumatic Bed"; 4,255,824 to Pertchik for "Cushion for Decubitus Ulcers"; 4,371,997 to Mattson for "Adjustable Firmness Cushion with Multiple Layered Foam-Filled Compartments": 4,494,260 to Olds et al. for "Body Support": and 4,534,078 to Viesturs et al. for "Body Supporting Mattress". These systems disclose generally uniform support surfaces, pressure isolating cell design, or mechanical pressure cycling.
All of the above patient support systems provide generally uniform support over an entire mattress or at least over broad sections. These systems relieve pressure locally on the skin and fatty tissue but do not relieve pressure deep into the muscle tissue adjacent the bony structures. Further, there is no disclosure of applying pressure at more than the accepted maximum capillary blood pressure of about 32 mm Hg. Even the air wave systems provide general support to areas supported by minimum or maximum inflated air cells. In none of these systems then is there a system that effectively removes the weight from selected body areas that are subject to high pressure or that claims to apply a pressure at the skin surface greater than the maximum capillary blood pressure level.
While the prevailing commercial understanding of tissue trauma does assume that the pressure at the skin surface represents pressures throughout the tissues and that long term even pressure is preferred, clinical research proves the opposite.
In 1953 Husain ("An Experimental Study of Some Pressure Effects on Tissues, with Reference to the Bed-Sore Problem", J. Path. Bact., Vol. 66, 1953, pp. 347-358) established "[s]upport for the modern theory that associates bed sores with initial deep lesions within muscles close to a bony surface or projection is based on two main sources of evidence: (1) pathological studies which have demonstrated muscular lesions long before superficial bed sores appear, or have shown that muscles lesions, recent or old, very often accompany bed sores; (2) experimental investigations into the mode of action of pressure, which have shown the susceptibility of muscle to physical disturbance as contrasted with the relative resistance to skin and to a lesser extent of fat . . . " (pp. 353,356)
Husain concluded:
1. "Pressure evenly distributed over a wide area of the body is much less damaging to the tissues than localized or point pressure." (p. 356)
2. "Low pressure maintained for long periods of time produces more tissue damage than high pressure for short periods." (p. 356)
3. "The time factor is thus more important than pressure intensity." (p. 356)
4. "Histological evidence of muscle damage can be demonstrated in the tissues deep to human bed sores. This appears to be the result of prolonged pressure rather than infection and almost certainly precedes the bed sores." (p. 357)
In 1960 Kosiak ("The Etiology of Decubitus Ulcers", Archives of Physical Medicine & Rehabilitation, January, 1961, pp. 19-28) experimented and determined:
1. "The application of alternating pressures, whereby the tissue was completely free of pressure for five minute intervals, showed consistently less change or no change when compared with tissue subjected to an equivalent amount of constant pressure. This was true even at pressures as high as 240 mm Hg. for three hours." (p. 28)
2. "Even when excessive pressures are applied for a sufficient period of time to result in early degenerative changes, it would appear that complete relief of pressure may often permit restoration of circulation and cellular metabolism without ulceration." (p. 28)
3. "Skeletal muscle from both normal and paraplegic rats exhibited a high degree of susceptibility to low constant pressure for relatively short periods of time." (p. 28)
4. "Microscopic pathologic changes in muscle were absent or less prominent following the application of equal amounts of alternating pressures in both normal and paraplegic rats." (p. 28)
During a study conducted at the Stanford University Medical Center, Stanford, Calif. the pressure inside of living tissue between the skin and bony protuberances of subjects was measured, apparently for the first time in history. This study and the conclusions reached are described by Le et al. in "An In-Depth Look at Pressure Sores Using Monolithic Silicon Pressure Sensors", Plastic and Reconstructive Surgery, December 1984, pp. 745-754.
"The most significant result from this investigation is that although the surface pressure may stay below the capillary pressure (25-35 mm Hg) the internal pressure may be several (three to five) times greater, which is theoretically large enough to cause pressure sores if unrelieved."
The conclusion of the Stanford study was that the highest pressures observed were adjacent the bone and that the pressure decreased with distance from the bone. Thus, the pressure on the skin was not the highest pressure observed. This is a result similar to the application of a force by a plate having a large surface area (analogous to the skin surface) to one side of the sponge (analogous to body tissue). Another plate with a small surface area (analogous to a bony protuberance) placed against the other side of the sponge resists the application of force. With equal forces on both plates, the pressure per unit area is much higher on the smaller plate. As a result, it was concluded that pressure sores originate near the bone and progress outwardly, eventually reaching the skin. This is contrary to conventional knowledge as described in the article entitled "Decubitus: A Persistent Problem" mentioned above, and does not appear to have received acceptance in the field and is not applied in commercial devices.
On page 753 of the article the authors concluded: "An important inference from this result is that the prevention of pressure sores must entail the removal of the load from the weight-bearing bony prominence rather than merely relief of local pressure at the skin underlying the prominence." The authors further project as an example of the application of their conclusions, that large-scale load removal could be accomplished by periodically dropping either side of a wheel chair seat from under the buttock on that side.
This proposed solution would result in the individual sitting in the wheel chair getting thrown against the arm of the wheel chair on the side from which pressure is removed. Thus, a practical solution of how to actually prevent pressure sores in a commercially viable product has not been designed or conceived. Further, how the application of such a theory would be applied to the much more prevalent bed support system has not heretofore been conceived.
Thus over the past three decades independent research teams have supported the following conclusions relating to bed sores.
1. Pressure within the tissues is not uniform.
2. Pressure is three to five times higher within the tissues than at the skin surface.
3. Muscle tissue which surrounds the bone structure is far more susceptible to damage than fat or skin.
4. Damage deep within the muscle tissue does precede a visible pressure sore at the skin.
5. Low pressure maintained for long periods of time produces more tissue damage than high pressure for short periods.
6. The time factor is thus more important than pressure intensity.